This invention relates to an alignment assembly for photomasking the back side of a semiconductor wafer relative to its front side, and to a method of using this assembly to deposit a metallic coating on selected areas of the wafer back side. More particularly, this invention relates to an assembly in which a photomask for the back side of a semiconductor wafer is automatically registered with particular regions on the wafer front side by nesting a pattern of ridges on the wafer front side with grooves in a distinctive alignment chuck.
In some applications, it is desirable to have a metallic coating on the collector region or back side of discrete semiconductor dies. These metallic coatings serve as solderable areas, or electrodes, by which a low resistance electrical connection can be made to the collector region. In most cases, it is economically advantageous to apply such a coating to the discrete dies before they are separated from the wafer in which they are made.
One widely used method of separating the discrete dies from the wafer involves chemical etching. In this method, one etches a grid pattern completely through the wafer, thus producing a plurality of discrete dies. In such instance, it is convenient for one to cover the entire back side of the wafer with a blanket nickel coating. One then etches completely through the wafer and the nickel coating to form a plurality of discrete dies with each die having a nickel coating on its back side. This may be satisfactory for dies containing planar type devices.
On the other hand, we have noted that if the dies contain mesa devices, the collector-base junctions of the mesa dies are exposed during etching. We also noted that since part of the nickel coating is etched away during dicing, it contaminates the etch solution. The dissolved nickel can deposit on the exposed collector-base junctions resulting in an instability in the junction. The present invention prevents such metallic ions from contaminating the etch solution, and yet permits one to use the conventional etch-out technique of separating nickel coated dies from a wafer. More particularly, the present invention provides an inexpensive and reliable means for selectively depositing the nickel coating outside the grid etch-out lines.
In the U.S. Pat. No. 3,841,261 Hudson et al, it was disclosed that one could form a pattern of ridges on the emitter face of a semiconductor wafer during the same process in which mesa emitters were formed. The above-mentioned patent application utilizes this pattern of ridges in connection with a wax spray mask for the front side of the wafer. The spray mask has grooves which correspond to the pattern of ridges. The spray mask interlocks with the wafer and allows selective deposition of wax on the mesa emitters on the front side of the semiconductor wafer.
This present invention makes use of these same wafer ridges to facilitate precise registry of selected regions in a photomask for the back side of the wafer with emitter regions on the front side of the wafer. In this way, a metallic coating can be applied to selected areas on the back side of the wafer between the etch-out grid lines and avoid etching the nickel coating during die separation.